Air pollution obscures the view of the London eye in central London.
Photograph: Ben Fathers/AFP/Getty Images

The government’s plan for tackling the UK’s air pollution crisis has been judged illegally poor at the high court, marking the second time in 18 months that ministers have lost in court on the issue.

The defeat is a humiliation for ministers who by law must cut the illegal levels of nitrogen dioxide suffered by dozens of towns and cities in the “shortest possible time”.

Legal NGO ClientEarth, which brought the case, argued that current plans ignore many measures that could help achieve this, placing too much weight on costs. On Wednesday Mr Justice Garnham agreed. He also said ministers knew that over-optimistic pollution modelling was being used, based on flawed lab tests of diesel vehicles rather than actual emissions on the road.

The government said it would not appeal against the decision and agreed in court to discuss with ClientEarth a new timetable for more realistic pollution modelling and the steps needed to bring pollution levels down to legal levels. The parties will return to court in a week but if agreement cannot be reached, the judge could impose a timetable upon the government.

At prime minister’s questions, Theresa May indicated that the government would respond positively, with new proposals: “We now recognise that Defra [the Department of Environment, Food and Rural Affairs] has to look at the judgement made by the courts and we now have to look again at the proposals we will bring forward. Nobody in this house doubts the importance of the issue of air quality. We have taken action, there is more to do and we will do it.”

Make central London diesel-free to solve air pollution crisis – report

Read more

Air pollution causes 50,000 early deaths and £27.5bn in costs every year, according to the government’s own estimates, and was called a “public health emergency” by MPs in April.

James Thornton, CEO of ClientEarth, said: “The time for legal action is over. I challenge Theresa May to take immediate action now to deal with illegal levels of pollution and prevent tens of thousands of additional early deaths in the UK. The high court has ruled that more urgent action must be taken. Britain is watching and waiting, prime minister.”

He said the increased action required would very likely include bigger and tougher clean air zones in more cities and other measure such as scrappage schemes for the dirtiest vehicles: “The government will have to be tougher on diesel.”

The mayor of London, Sadiq Khan, who took part in the case against the government, said: “Today’s ruling lays the blame at the door of the government for its complacency in failing to tackle the problem quickly and credibly. In so doing they have let down millions of people the length and breadth of the country.”

A spokeswoman for Defra said: “Improving air quality is a priority for this government and we are determined to cut harmful emissions. Our plans have always followed the best available evidence - we have always been clear that we are ready to update them if necessary. Whilst our huge investment in green transport initiatives and plans to introduce clean air zones [in six cities] around the country will help tackle this problem, we accept the court’s judgment. We will now carefully consider this ruling, and our next steps, in detail.”

ClientEarth defeated the government on the same issue at the supreme court in April 2015. Ministers were then ordered to draw up a new action plan, but now that new plan has also been found to be illegal.

Documents revealed during the latest case showed the Treasury had blocked plans to charge diesel cars to enter towns and cities blighted by air pollution, concerned about the political impact of angering motorists. Both the environment and transport departments recommended changes to vehicle excise duty rates to encourage the purchase of low-pollution vehicles, but the Treasury also rejected that idea.

Documents further showed that the government’s plan to bring air pollution down to legal levels by 2020 for some cities and 2025 for London had been chosen because that was the date ministers thought they would face European commission fines, not which they considered “as soon as possible”.

There had been a draft government plan for 16 low emission zones, which polluting vehicles are charged to enter, in cities outside London but the number was cut to just five on cost grounds.

All these proposals will now be revisited. Thornton said a national network of clean air zones needed to be in place by 2018. “If you put in clean air zones, it works overnight.”

Dr Penny Woods, chief executive of the British Lung Foundation, said: “We urgently need a new clean air act that restricts the most polluting vehicles from our urban areas and protects everyone’s lung health – air pollution affects all of us.”

Sam Hall, at conservative thinktank Bright Blue, said there should be more power and funding devolved to local authorities to enable all English cities to set up clean air zones and more support for electric cars.

Keith Taylor, Green party MEP, said: “The failure highlighted by the judge today is as much moral as it is legal: ministers have displayed an extremely concerning attitude of indifference towards their duty to safeguard the health of British citizens.”